Leukemia starts with DNA damage in a single blood cell inside your bone marrow. That damage disrupts the normal instructions telling the cell when to grow, when to mature, and when to die. The result is a cell that copies itself uncontrollably, flooding the bone marrow and bloodstream with defective cells that crowd out the healthy ones your body needs to function.
Where Blood Cells Go Wrong
Your bone marrow constantly produces new blood cells through a tightly regulated life cycle. Immature cells, called blasts, are supposed to develop into functioning red blood cells, white blood cells, or platelets, then eventually die to make room for fresh replacements. Leukemia disrupts this cycle at one of two points.
In acute leukemia, the disruption happens early. Immature blast cells stop developing altogether and never gain any useful function. They’re essentially stuck as “baby cells” that can’t do anything but multiply. In chronic leukemia, the disruption happens later. The cells do mature, and they can still partially function for a while, but they never receive the signal to die. Over time, these long-lived cells accumulate and cause problems of their own. This distinction is why acute leukemia tends to progress rapidly and feel sudden, while chronic leukemia can simmer undetected for months or years before symptoms appear.
The DNA Mutations That Drive It
The damage that turns a normal blood cell into a leukemic one is genetic, but that doesn’t necessarily mean it was inherited. Most of the time, the mutations happen during a person’s lifetime, either from environmental exposures or from random copying errors when cells divide. A single mutation is rarely enough. Leukemia typically requires several genetic changes to accumulate in the same cell line before things go haywire.
One well-studied example involves a swap of genetic material between two chromosomes (chromosomes 9 and 22). When pieces of these chromosomes break off and reattach to each other, they create an abnormal fused gene that produces a protein driving constant cell growth. This particular rearrangement, known as the Philadelphia chromosome, is found in nearly all cases of chronic myeloid leukemia. It essentially flips a permanent “on” switch for cell division. Other types of leukemia involve different mutations, but the principle is the same: damaged DNA removes the brakes on cell growth, blocks normal maturation, or prevents cells from dying on schedule.
What Damages the DNA in the First Place
Several environmental exposures are known to increase the risk of leukemia by damaging cell DNA directly. High levels of ionizing radiation, the type found in X-rays, nuclear fallout, and radon gas, can break DNA strands and trigger the mutations that lead to uncontrolled growth. This was documented extensively in survivors of atomic bomb exposure and in people who received high-dose radiation therapy for earlier cancers.
Chemical exposures matter too. Benzene, a compound found in gasoline, industrial solvents, and cigarette smoke, is one of the most established chemical risk factors. Certain chemotherapy drugs used to treat other cancers can paradoxically damage bone marrow DNA enough to cause leukemia years later. Tobacco smoke, some petrochemicals, and specific pesticides have also been linked to increased risk. For children, a mother’s exposure to pesticides and certain chemicals during pregnancy may raise the child’s chances of developing acute lymphocytic leukemia, as can exposure to these chemicals during early childhood.
That said, many people who develop leukemia have no identifiable exposure. Random errors during normal cell division account for a significant share of cases, which is part of why leukemia can strike people with no obvious risk factors.
Inherited Risk Factors
While most leukemia isn’t directly inherited, recent estimates suggest that more than 10% of all blood cancers may have a genetic component passed down through families. In children specifically, studies indicate that 10% to 30% of cases are caused by underlying inherited gene variants.
Several well-known genetic conditions increase leukemia risk substantially. Children with Down syndrome (who carry an extra copy of chromosome 21) have a significantly elevated risk of developing acute leukemia in childhood. Li-Fraumeni syndrome, caused by mutations in a key tumor-suppressing gene, predisposes people to a range of cancers including leukemia. Fanconi anemia, a rare inherited condition affecting DNA repair, carries a high risk of bone marrow failure and leukemia, often appearing in childhood. Other conditions involving defects in telomere biology (the protective caps on chromosomes) or in genes that regulate blood cell development can also set the stage for leukemia across a wide age range.
For families where multiple relatives have been diagnosed with blood cancers, genetic testing can sometimes identify whether an inherited variant is involved, which helps guide monitoring for at-risk family members.
How Leukemia Takes Over the Bone Marrow
Once leukemic cells gain a growth advantage, they don’t just passively accumulate. They actively reshape the bone marrow environment to favor their own survival. Leukemia cells infiltrate the spaces where normal blood stem cells live and hijack the signals those stem cells depend on. Research has shown that leukemic cells release chemical signals that suppress the activity of bone-building cells in the marrow, weakening the support system that healthy blood production relies on. In effect, the leukemia converts the bone marrow from a healthy blood cell factory into an environment that’s permissive of cancer growth and hostile to normal cell development.
This hostile takeover is what produces leukemia’s hallmark symptoms. As leukemic blasts crowd out healthy cells, the body loses its ability to make enough red blood cells (causing anemia and fatigue), platelets (causing easy bruising and bleeding), and functional white blood cells (causing frequent infections). A diagnosis of acute leukemia is typically made when 20% or more of the cells in the bone marrow are immature blasts, a threshold that reflects how thoroughly the disease has displaced normal blood production.
Why Some Types Progress Faster Than Others
The speed of leukemia depends largely on which cell type is affected and where in its development the mutation strikes. Acute myeloid leukemia and acute lymphocytic leukemia both involve blasts that are frozen at an immature stage, dividing rapidly and offering no useful function. Without treatment, acute leukemias can become life-threatening within weeks because the bone marrow gets overwhelmed quickly.
Chronic leukemias behave differently. In chronic lymphocytic leukemia, for instance, the abnormal cells are more mature and accumulate slowly. People can live years without symptoms, sometimes discovering the disease only through a routine blood test. Chronic myeloid leukemia, driven by the Philadelphia chromosome, also progresses gradually in its early phase but can accelerate into a crisis that resembles acute leukemia if untreated.
The type of leukemia also determines how the body responds to treatment. Because acute leukemia cells divide so rapidly, they’re often more vulnerable to treatments that target fast-dividing cells. Chronic forms, with their slower-growing and partially functional cells, sometimes require different strategies, including drugs designed to block the specific proteins produced by the underlying genetic mutation.